1. Field of the Invention
The present invention generally relates to an apparatus and method for transmitting/receiving a signal in a mobile communication system, and more particularly to an apparatus and method for transmitting/receiving a signal that can reduce a peak to average power ratio (PAPR) and decoding complexity in a mobile communication system using a multiple-input multiple-output (MIMO) scheme.
2. Description of the Related Art
One of the most fundamental problems in mobile communication is to efficiently and reliably transmit data on a channel. In the next-generation multimedia mobile communication systems on which extensive research is being conducted, high-speed communication systems are required which can process and transmit various types of information such as images, wireless data, and the like beyond the initial voice-centric service. Thus, it is essential to increase system efficiency using channel coding suitable for the high-speed communication systems.
Unfortunately, errors and information loss may occur due to many factors such as multipath interference, shadowing, propagation attenuation, time variant noise, interference fading and the like in wireless channel environments of the wireless communication systems.
An actual transmitted signal may be significantly distorted due to the information loss. This distortion may be a factor degrading the overall performance of a wireless communication system. Various error control techniques according to channel characteristics are used to reduce the information loss. Among the error control techniques, one technique uses error-correcting codes.
In addition, diversity schemes are used to eliminate communication instability due to the fading effect. The diversity schemes can be classified into a time diversity scheme, a frequency diversity scheme and an antenna diversity scheme, that is, a space diversity scheme.
The antenna diversity scheme uses multiple antennas and is divided into a receive (Rx) antenna diversity scheme using multiple Rx antennas, a transmit (Tx) antenna diversity scheme using multiple Tx antennas, and a multiple-input multiple-output (MIMO) scheme using multiple Rx antennas and multiple Tx antennas.
A kind of the MIMO scheme is a space-time coding (STC) scheme. The STC scheme extends time-domain coding to a space domain and achieves a lower error rate by transmitting signals encoded in a predefined coding scheme using multiple Tx antennas.
Vahid Tarokh, et al. proposed space-time block coding (STBC) as one method of efficiently applying the antenna diversity scheme (see Vahid Tarokh, et al., “Space-Time Block Coding from Orthogonal Designs”, IEEE Trans. on Info., Theory, Vol. 45, pp. 1456-1467, July 1999). The STBC scheme is an extension of the Tx antenna diversity scheme of S. M. Alamouti for two or more Tx antennas (see S. M. Alamouti, “A Simple Transmitter Diversity Technique for Wireless Communications”, IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, October 1988).
FIG. 1 illustrates a structure of a signal transmitter of a MIMO mobile communication system using STBC and four Tx antennas proposed by Vahid Tarokh.
Referring to FIG. 1, the signal transmitter includes a modulator 100, a serial-to-parallel (S/P) converter 102, a space-time block encoder 104, and four Tx antennas, that is, a first Tx antenna (Tx. ANT 1) 106 to a fourth Tx antenna (Tx. ANT 4) 112.
When information/data bits are input, the modulator 100 generates modulated symbols by modulating the input information/data bits according to a modulation scheme and then outputs the modulated symbols to the S/P converter 102. The modulation scheme can use, for example, any one of binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), pulse amplitude modulation (PAM), phase shift keying (PSK), and the like.
The S/P converter 102 receives and parallel converts the modulated symbols serially output from the modulator 100 and then outputs the parallel converted modulated symbols to the space-time block encoder 104. It is assumed that the modulated symbols serially output from the modulator 100 are s1, s2, s3 and s4. Using an STBC process, the space-time block encoder 104 encodes the four modulated symbols, that is, s1, s2, s3 and s4, input from the S/P converter 102 and then outputs the following encoded modulated symbols.
      G    4    =      [                                        s            1                                                s            2                                                s            3                                                s            4                                                            -                          s              2                                                            s            1                                                -                          s              4                                                            s            3                                                            -                          s              3                                                            s            4                                                s            1                                                -                          s              2                                                                        -                          s              4                                                            -                          s              3                                                            s            2                                                s            1                                                            s            1            *                                                s            2            *                                                s            3            *                                                s            4            *                                                            -                          s              2              *                                                            s            1            *                                                -                          s              4              *                                                            s            3            *                                                            -                          s              3              *                                                            s            4            *                                                s            1            *                                                -                          s              2              *                                                                        -                          s              4              *                                                            -                          s              3              *                                                            s            2            *                                                s            1            *                                ]  
G4 denotes a coding matrix of symbols to be transmitted via the four Tx antennas 106 to 112. In the coding matrix, elements of each row are mapped to the Tx antennas and elements of each column are mapped to the Tx antennas in the associated time intervals.
In the first time interval, s1, s2, s3 and s4 are transmitted via the first, second, third and fourth Tx antennas 106, 108, 110 and 112, respectively. In the eighth time interval, −s*4, −s*3, s*2 and s*1 are transmitted via the first, second, third and fourth Tx antennas 106, 108, 110 and 112, respectively.
As described above, the space-time block encoder 104 controls the input modulated symbols to be transmitted via the four Tx antennas for the eight time intervals by applying negative and conjugate operations to the input modulated symbols. Since the symbols transmitted via the four Tx antennas are mutually orthogonal, a diversity gain equal to the diversity order may be achieved.
The structure of the signal transmitter of the MIMO mobile communication system using the STBC and the four Tx antennas proposed by Vahid Tarokh has been described with reference to FIG. 1.
FIG. 2 illustrates the structure of the signal receiver mapped to the structure of the signal transmitter of FIG. 1.
Referring to FIG. 2, the signal receiver is provided with multiple antennas, for example, P number of Rx antennas, that is, a first Rx antenna (Rx. ANT 1) 200 to a P-th Rx antenna (Rx. ANT P) 202, a channel estimator 204, a signal combiner 206, a detector 208, a parallel-to-serial (P/S) converter 210, and a demodulator 212. In FIG. 2, it is assumed that the number of Tx antennas of the signal transmitter mapped to the signal receiver is different from that of Rx antennas of the signal receiver. Alternatively, the number of Tx antennas of the signal transmitter can be the same as that of Rx antennas of the signal receiver.
First, the first Rx antenna 200 to the P-th Rx antenna 202 receive signals transmitted via the four Tx antennas from the signal transmitter as described with reference to FIG. 1. The signals received by the first Rx antenna 200 to the P-th Rx antenna 202 are output to the channel estimator 204 and the signal combiner 206.
The channel estimator 204 estimates channel coefficients representing channel gains and outputs the channel coefficients to the detector 208 and the signal combiner 206. The signal combiner 206 combines the signals received via the first Rx antenna 200 to the P-th Rx antenna 202 with the channel coefficients output from the channel estimator 204, and outputs received symbols to the detector 208.
The detector 208 generates hypothesis symbols by multiplying the received symbols output from the signal combiner 206 by the channel coefficients output from the channel estimator 204, computes decision statistics for all possible symbols transmitted from the signal transmitter using the hypothesis symbols, detects modulated symbols transmitted from the signal transmitter through threshold detection, and outputs the modulated symbols to the P/S converter 210.
The P/S converter 210 receives and serially converts the modulated symbols parallel output from the detector 208 and outputs the serially converted modulated symbols to the demodulator 212. The demodulator 212 receives the serially converted modulated symbols output from the P/S converter 210 and recovers the original information bits by demodulating the symbols in a demodulation scheme mapped to a modulation scheme applied to the modulator 100 of the signal transmitter.
As described above, the S. M. Alamouti proposed STBC scheme is advantageous in that a diversity order equal to the product of the number of Tx antennas and the Rx antennas, that is, full diversity order, may be achieved without degrading a data rate even when the signal transmitter transmits complex symbols via two Tx antennas.
In the structures of the signal transmitter and receiver of FIGS. 1 and 2 based on the Vahid Tarokh scheme extended from S. M. Alamouti proposed STBC scheme, the full diversity order may be achieved using space-time block codes in the form of a matrix with orthogonal columns. However, there is a problem in that a data rate may be reduced to ½ since four complex symbols are transmitted for eight time intervals. In addition, there is a problem in that reception performance may be degraded in a fast fading channel environment since eight time intervals are required to transmit one signal block, that is, four symbols.
When signals are transmitted via four or more Tx antennas in the STBC scheme as described above, (2×N) number of time intervals are required to transmit N symbols. In this case, there are problems in that latency may be lengthened and a data rate may be degraded. That is, when the STBC scheme is used, it is impossible to achieve full diversity and full rate (FDFR).